This disclosure generally pertains to reduced weight and density gypsum panels with improved thermal insulation properties, heat shrinkage resistance, and fire resistance.
Gypsum panels typically used in building and other construction applications (such as a gypsum wallboard or ceiling panels) typically comprise a gypsum core with cover sheets of paper, fiberglass or other suitable materials. Gypsum panels typically are manufactured by mixing calcined gypsum, or “stucco,” with water and other ingredients to prepare a slurry that is used to form the core of the panels. As generally understood in the art, stucco comprises predominately one or more forms of calcined gypsum, i.e. gypsum subjected to dehydration (typically by heating) to form anhydrous gypsum or hemihydrate gypsum (CaSO4.1/2H2O). The calcined gypsum may comprise beta calcium sulfate hemihydrate, alpha calcium sulfate hemihydrate, water-soluble calcium sulfate anhydrite, or mixtures of any or all of these, from natural or synthetic sources. When introduced into the slurry, the calcined gypsum begins a hydration process which is completed during the formation of the gypsum panels. This hydration process, when properly completed, yields a generally continuous crystalline matrix of set gypsum dihydrate in various crystalline forms (i.e. forms of CaSO4.2H2O).
During the formation of the panels, cover sheets typically are provided as continuous webs. The gypsum slurry is deposited as a flow or ribbon on a first of the cover sheets. The slurry is spread across the width of the first cover sheet at a predetermined approximate thickness to form the panel core. A second cover sheet is placed on top of the slurry, sandwiching the gypsum core between the cover sheets and forming a continuous panel.
The continuous panel typically is transported along a conveyer to allow the core to continue the hydration process. When the core is sufficiently hydrated and hardened, it is cut to one or more desired sizes to form individual gypsum panels. The panels are then transferred into and passed through a kiln at temperatures sufficient to dry the panels to a desired free moisture level (typically relatively low free moisture content).
Depending on the process employed and the expected use of the panels and other such considerations, additional slurry layers, strips or ribbons comprising gypsum and other additives may be applied to the first or second cover sheets to provided specific properties to the finished panels, such as hardened edges or a hardened panel face. Similarly, foam may be added to the gypsum core slurry and/or other slurry strips or ribbons at one or more locations in the process to provide a distribution of air voids within the gypsum core or portions of the core of the finished panels.
The resulting panels may be further cut and processed for use in a variety of applications depending on the desired panel size, cover layer composition, core compositions, etc. Gypsum panels typically vary in thickness from about ¼ inch to about one inch depending on their expected use and application. The panels may be applied to a variety of structural elements used to form walls, ceilings, and other similar systems using one or more fastening elements, such as screws, nails and/or adhesives.
Should the finished gypsum panels be exposed to relatively high temperatures, such as those produced by high temperature flames or gases, portions of the gypsum core may absorb sufficient heat to start the release of water from the gypsum dihydrate crystals of the core. The absorption of heat and release of water from the gypsum dihydrate may be sufficient to retard heat transmission through or within the panels for a time. The gypsum panel can act as a barrier to prevent high temperature flames from passing directly through the wall system. The heat absorbed by the gypsum core can be sufficient to essentially recalcine portions of the core, depending on the heat source temperatures and exposure time. At certain temperature levels, the heat applied to a panel also may cause phase changes in the anhydrite of the gypsum core and rearrangement of the crystalline structures. In some instances, the presence of salts and impurities may reduce the melting point of the gypsum core crystal structures.
Gypsum panels may experience shrinkage of the panel dimensions in one or more directions as one result of some or all of these high temperature heating effects, and such shrinkage may cause failures in the structural integrity of the panels. When the panels are attached to wall, ceiling or other framing assemblies, the panel shrinkage may lead to the separation of the panels from other panels mounted in the same assemblies, and from their supports, and, in some instances, collapse of the panels or the supports (or both). As a result, high temperature flames or gases may pass directly into or through a wall or ceiling structure.
Gypsum panels have been produced that resist the effects of relatively high temperatures for a period of time, which may inherently delay passage of high heat levels through or between the panels, and into (or through) systems using them. Gypsum panels referred to as fire resistant or “fire rated” typically are formulated to enhance the panels' ability to delay the passage of heat though wall or ceiling structures and play an important role in controlling the spread of fire within buildings. As a result, building code authorities and other concerned public and private entities typically set stringent standards for the fire resistance performance of fire rated gypsum panels.
The ability of gypsum panels to resist fire and the associated extreme heat may be evaluated by carrying out generally-accepted tests. Examples of such tests are routinely used in the construction industry, such as those published by Underwriters Laboratories (“UL”), such as the UL U305, U419 and U423 test procedures and protocols, as well as procedures described in the specifications E119 published by the American Society for Testing and Materials (ASTM). Such tests may comprise constructing test assemblies using gypsum panels, normally a single-layer application of the panels on each face of a wall frame formed by wood or steel studs. Depending on the test, the assembly may or may not be subjected to load forces. The face of one side of the assembly, such as an assembly constructed according to UL U305, U419 and U423, for example, is exposed to increasing temperatures for a period of time in accordance with a heating curve, such as those discussed in the ASTM E119 procedures.
The temperatures proximate the heated side and the temperatures at the surface of the unheated side of the assembly are monitored during the tests to evaluate the temperatures experienced by the exposed gypsum panels and the heat transmitted through the assembly to the unexposed panels. The tests are terminated upon one or more structural failures of the panels and/or when the temperatures on the unexposed side of the assembly exceed a predetermined threshold. Typically, these threshold temperatures are based on the maximum temperature at any one of such sensors and/or the average of the temperature sensors on the unheated side of the assembly.
Test procedures, such as those set forth in UL U305, U419 and U423 and ASTM E119, are directed to an assembly's resistance to the transmission of heat through the assembly as a whole. The tests also provide, in one aspect, a measure of the resistance of the gypsum panels used in the assembly to shrinkage in the x-y direction (width and length) as the assembly is subjected to high temperature heating. Such tests also provide a measure of the panels' resistance to losses in structural integrity that result in opening gaps or spaces between panels in a wall assembly, with the resulting passage of high temperatures into the interior cavity of the assembly. In another aspect, the tests provide a measure of the gypsum panels' ability to resist the transmission of heat through the panels and the assembly. It is believed that such tests reflect the specified system's capability for providing building occupants and firemen/fire control systems a window of opportunity to address or escape fire conditions.
In the past, various strategies were employed to improve the fire resistance of fire rated gypsum panels. For example, thicker, denser panel cores have been provided which use more gypsum relative to less dense gypsum panels, and therefore include an increased amount of water chemically bound within the gypsum (calcium sulfate dihydrate), to act as a heat sink, to reduce panel shrinkage, and to increase the structural stability and strength of the panels. Alternatively, various ingredients including glass fiber and other fibers have been incorporated into the gypsum core to enhance the gypsum panel's fire resistance by increasing the core's tensile strength and by distributing shrinkage stresses throughout the core matrix. Similarly, amounts of certain clays, such as those of less than about one micrometer size, and colloidal silica or alumina additives, such as those of less than one micrometer size, have been used in the past to provide increased fire resistance (and high temperature shrink resistance) in a gypsum panel core. It has been recognized, however, that reducing the weight and/or density of the core of gypsum panels by reducing the amount of gypsum in the core will adversely affect the structural integrity of the panels and their resistance to fire and high heat conditions.
Another approach has been to add unexpanded vermiculite (also referred to as vermiculite ore) and mineral or glass fibers into the core of gypsum panels. In such approaches, the vermiculite is expected to expand under heated conditions to compensate for the shrinkage of the gypsum components of the core. The mineral/glass fibers were believed to hold portions of the gypsum matrix together.
Such an approach is described in U.S. Pat. Nos. 2,526,066 and 2,744,022, which discuss the use of comminuted unexfoliated vermiculate and mineral and glass fibers in proportions sufficient to inhibit the shrinkage of gypsum panels under high temperature conditions. Both references, however, relied on a high density core to provide sufficient gypsum to act as a heat sink. They disclose the preparation of ½ inch thick gypsum panels with a weight of, 2 to 2.3 pounds per square foot (2,000 to 2,300 pounds per thousand square feet (“lb/msf”)) and board densities of about 50 pounds per cubic foot (“pcf”) or greater.
The '066 patent reported that sections cut from such panels (with 2 percent mineral fiber and 7.5% of minus 28 mesh vermiculite) evidenced up to 19.1% thickness expansion when heated at 1400° F. (760° C.) for 30 minutes, but did not provide any information on the x-y direction shrinkage of those samples. The '066 patent further cautioned that, depending on the panel formulation and vermiculite content, vermiculite expansion could cause panel failures due to bulging panels and/or cracks and openings in the panels.
The '022 patent was directed at increasing the gypsum content (and thus density and weight) of the panels disclosed in the '066 patent by reducing the mineral/glass fiber content of those panels to provide a greater gypsum-heat sink capacity. References such as the '022 patent further recognized that the expansive properties of vermiculite, unless restrained, would result in spalling (that is, fragmenting, peeling or flaking) of the core and destruction of a wall assembly in a relatively short time at high temperature conditions.
In another example, U.S. Pat. No. 3,454,456 describes the introduction of unexpanded vermiculite into the core of fire rated gypsum wallboard panels to resist the shrinkage of the panels. The '456 patent also relies on a relatively high gypsum content and density to provide a desired heat sink capacity. The '456 patent discloses board weights for finished ½ inch gypsum panels of with a minimum weight of about 1925 lb/msf, and a board density of about 46 pcf. This is a density comparable to thicker and much heavier ⅝ inch thick gypsum panels (about 2400 lb/msf) presently offered commercially for fire rated applications.
The '456 patent also discloses that using vermiculite in a gypsum panel core to raise the panel's fire rating is subject to significant limitations. For example, the '456 patent notes that the expansion of the vermiculite within the core may cause the core to disintegrate due to spalling and other destructive effects. The '456 patent also discloses that unexpanded vermiculite particles may so weaken the core structure that the core becomes weak, limp, and crumbly. The '456 patent purports to address such significant inherent limitations with the use of vermiculite in gypsum panels by employing a “unique” unexpanded vermiculite with a relatively small particle size distribution (more than 90% of the unexpanded particles smaller than a No. 50 mesh size (approximately 0.0117 inch (0.297 mm) openings), with less than 10% slightly larger than no. 50 mesh size). This approach purportedly inhibited the adverse effects of vermiculite expansion on the panel, as explained at col. 2, 11. 52-72 of the '456 patent.
The '456 patent, in addition, explains that the unexpanded vermiculite having the above described particle size distribution corresponds to a product known commercially as “Grade No. 5” unexpanded vermiculite. Grade No. 5 unexpanded vermiculite has been used in commercial fire resistant/fire rated panels with gypsum cores of conventional board densities (for example, from about 45 pcf to in excess of about 55 pcf) since at least the early 1970s. For the reasons discussed above, the use of unexpanded vermiculite comprising a significant distribution of particles with sizes larger than those typical of Grade No. 5 unexpanded vermiculite has been considered potentially destructive of fire resistance panels due to the above mentioned spalling and other effects caused by the expansion of the vermiculite within a gypsum core at high temperature conditions.
In another approach, U.S. Pat. No. 3,616,173 is directed to fire resistant gypsum panels with a gypsum core characterized by the '173 patent as a lighter weight or lower density. The '173 patent distinguished its panels from prior art ½ inch panels weighing about 2,000 lb/msf or more and having core densities in excess of about 48 pcf. Thus, the '173 patent discloses ½ inch thick panels with a density of at or above about 35 pcf, and preferably about 40 pcf to about 50 pcf. The '173 patent achieves its disclosed core densities by incorporating significant amounts of small particle size inorganic material of either clay, colloidal silica, or colloidal alumina in its gypsum core, as well as glass fibers in amounts required to prevent the shrinkage of its gypsum panels under high temperature conditions.
The '173 patent discloses the further, optional addition of unexpanded vermiculite to its gypsum core composition, along with the required amounts of its disclosed small particle size inorganic materials. Even with these additives, however, the disclosed testing of each of the '173 patent's panels showed that they experienced significant shrinkage. That shrinkage occurred notwithstanding the fact that each of the tested and disclosed panels had core densities of about 43 pcf or greater.
For ½ inch thick gypsum panels, the '173 patent's disclosed panels have a “shrink resistance” from about 60% to about 85%. “Shrink resistance” as used in the '173 patent is a measure of the proportion or percentage of the x-y (width-length) area of a segment of core that remains after the core is heated to a defined temperature over a defined period of time as described in the '173 patent. See, e.g., col. 12, 11. 41-49.
Other efforts also have been made to increase the strength and structural integrity of gypsum panels and reduce panel weight by various means. Examples of such light weight gypsum boards include, U.S. Pat. Nos. 7,731,794 and 7,736,720 and U.S. Patent Application Publication Nos. 2007/0048490 A1, 2008/0090068 A1, and 2010/0139528 A1.
Finally, it is noted that in the absence of water resistant additives, when immersed in water, set gypsum can absorb water up to 50% of its weight. And, when gypsum panels—including fire resistant gypsum panels—absorb water, they can swell, become deformed and lose strength which may degrade their fire-resistance properties. Low weight fire-resistant panels have far more air and/or water voids than conventional, heavier fire-resistant panels. These voids would be expected to increase the rate and extent of water uptake, with the expectation that such low weight fire-resistant panels would be more water absorbent than conventional heavier fire-resistant panels.
Many attempts have been made in the past to improve the water resistance of gypsum panels generally. Various hydrocarbons, including wax, resins and asphalt have been added to the slurries used to make gypsum panels in order to impart water resistance to the panels. Siloxanes also have been used in gypsum slurries imparting water resistance to gypsum panels by forming silicone resins in situ. Siloxanes, however, would not be expected to sufficiently protect low weight panels. Thus there is a need in the art for a method of producing low weight and density fire-resistant gypsum panels with improved water-resistance at reasonable cost by enhancing the water resistance normally imparted by siloxanes.